The nocebo effect represents a fascinating yet often underestimated phenomenon in clinical medicine and pharmacovigilance. It occurs when patients experience adverse symptoms solely because they expect them, rather than due to the pharmacological action of a drug. This review provides a comprehensive analysis of the nocebo effect, tracing its history from Walter Kennedy’s 1961 coinage of the term to the latest neurobiological and clinical research of 2024–2026.
We examine the psychological and neurobiological mechanisms underlying nocebo responses, including the roles of negative expectations, classical conditioning, and observational (social) learning. Recent systematic reviews and meta‑analyses are discussed to quantify the magnitude of nocebo effects and identify key risk factors. The profound implications for pharmacovigilance are highlighted: nocebo effects can mimic true adverse drug reactions, inflate adverse event data, confound signal detection, and lead to unnecessary treatment discontinuation. Strategies to mitigate nocebo effects are explored, focusing on optimized risk communication, positive social modelling, and patient‑provider interactions. Finally, the latest research, including neuroimaging studies and randomised controlled trials on vaccination and communication strategies, is reviewed to provide a forward‑looking perspective.
1. Introduction: The Power of Negative Expectation
Imagine a patient who reads the long list of potential side effects on a medication leaflet and, within days, begins to experience nausea, fatigue, and dizziness—symptoms that disappear when the drug is stopped but reappear when the same inert capsule is given under the same name. This is not a case of a hidden drug intolerance but a manifestation of the nocebo effect: adverse outcomes triggered by negative expectations rather than by the pharmacological activity of the treatment itself.
While the placebo effect—the beneficial counterpart—has been widely studied and celebrated, the nocebo effect has historically received less attention. Yet its clinical and public health impact is substantial. A large‑scale review of pharmacological clinical trials involving over half a million participants found that nearly three‑quarters of placebo‑treated individuals reported side effects, often at rates comparable to those receiving the active drug.
Understanding the nocebo effect is therefore not merely an academic exercise; it is essential for improving drug safety monitoring, enhancing patient care, and ensuring that effective therapies are not abandoned due to misattributed symptoms.
This review synthesises current knowledge on the nocebo effect, with a particular focus on its relevance to pharmacovigilance. We begin by defining the phenomenon and tracing its historical roots, then explore the psychological and neurobiological mechanisms that drive nocebo responses. Recent meta‑analyses and empirical studies are examined to quantify the magnitude of nocebo effects and identify key risk factors. The core section of this article addresses the critical implications for pharmacovigilance: how nocebo effects can distort adverse event reporting, confound signal detection, and lead to unnecessary treatment discontinuation. Finally, evidence‑based strategies for mitigating nocebo effects—through optimised risk communication, positive social modelling, and enhanced patient‑provider interactions—are presented, along with future research directions.
2. Definition and Historical Background
2.1 What Is the Nocebo Effect?
The nocebo effect is defined as adverse outcomes that arise due to negative expectations rather than the pharmacological activity of an intervention. A concise working definition is: “the occurrence of a harmful event that stems from consciously or subconsciously expecting it”. Nocebo responses refer specifically to negative outcomes in clinical trials or practice that cannot be explained by the treatment’s pharmacologic effects.
Crucially, the nocebo effect is not merely a psychological curiosity but a genuine neurobiological phenomenon. It can cause real harm, interfere with treatment adherence, and distort the assessment of drug safety in both clinical trials and post‑marketing surveillance.
2.2 Historical Roots
The term “nocebo” was introduced in 1961 by Walter P. Kennedy, a British physician who sought a counterpart to the well‑established “placebo.” Kennedy derived “nocebo” from the Latin nocere—meaning “to harm”—and defined it as the negative counterpart of the positive placebo reaction. In his seminal paper, Kennedy presciently noted: “One wonders how often a useful drug has been discarded because of ‘toxic effect’ in the first trials which have been due to the accident of there being an appreciable number of nocebo reactors in the test subjects”.
For decades, the nocebo effect remained overshadowed by its more celebrated “twin.” It is only in recent years that researchers, clinicians, and regulators have begun to appreciate its pervasive influence on clinical outcomes and drug safety data.
3. Mechanisms of the Nocebo Effect
Nocebo effects arise through a combination of psychological and neurobiological pathways, primarily involving negative expectations, classical conditioning, and observational (social) learning.
3.1 Negative Expectations (Verbal Suggestions)
The most direct route to a nocebo effect is through verbal information that creates negative expectations. Telling a patient that a drug may cause nausea, headache, or fatigue can, by itself, trigger those very symptoms. This is not merely a matter of suggestion; it reflects a real neurobiological response. For example, patients informed about possible gastrointestinal side effects of a drug reported significantly more nausea, even when given a placebo.
The power of verbal suggestion is so strong that it can even override the actual pharmacological effects of a drug. In one classic study, patients who were told that a placebo was a stimulant experienced increased heart rate and blood pressure, while those told it was a sedative experienced the opposite—despite receiving the same inert substance.
3.2 Classical Conditioning
Classical conditioning is another powerful mechanism. If a patient has previously experienced a genuine adverse reaction to a drug (or even to a similar‑looking pill), the mere act of taking a medication—or being in a medical setting—can trigger the same symptoms, even if the new drug is pharmacologically inert. This is the same learning process by which a dog salivates at the sound of a bell previously paired with food.
In the context of nocebo effects, a neutral stimulus (e.g., a white pill) becomes associated with an unpleasant experience (e.g., nausea caused by a chemotherapy drug). Over time, the pill alone can elicit the nausea, even when it contains only a placebo.
3.3 Observational (Social) Learning
Humans are inherently social learners: we update our beliefs and behaviours based on observing others’ experiences. This adaptive capacity can, however, backfire. Watching another person experience side effects—whether in a clinical trial, a waiting room, or a social media video—can induce the same symptoms in the observer, even without direct exposure to the drug.
A 2024 systematic review and meta‑analysis by Saunders and colleagues examined this phenomenon. The analysis included 20 studies (total n = 1,388) and found that social learning induced a medium‑large effect size (Hedges’ g = 0.74) for nocebo effects compared to no treatment. The effect was similar in magnitude to classical conditioning and greater than explicit instruction (g = 0.46). Factors that strengthened socially induced nocebo effects included face‑to‑face social modelling, longer exposure, higher proportions of female participants and models, and greater observer empathy.
3.4 Neurobiological Substrates
Advances in neuroimaging have begun to reveal the brain networks underlying nocebo effects. Nocebo hyperalgesia (increased pain perception due to negative expectations) has been linked to activity in the hippocampus, amygdala, and prefrontal cortex—regions involved in threat processing, anxiety, and pain modulation.
A 2024 study published in Scientific Reports investigated the persistence of placebo and nocebo effects over one week using functional magnetic resonance imaging (fMRI). The researchers found that both positive and negative pain‑modulation effects persisted over time and did not undergo extinction. Importantly, the persistence of nocebo effects was specifically predicted by enhanced thalamus activity during the first session, while placebo effects were linked to reduced amygdala activity and enhanced activity in the anterior insula and dorsolateral prefrontal cortex. These findings suggest that nocebo and placebo effects are mediated by distinct neural circuits, a discovery with implications for developing targeted interventions.
4. Risk Factors for Nocebo Effects
A 2024 meta‑review (review of reviews) by Grosso and colleagues identified nine categories of risk factors associated with nocebo effects:
| Risk Factor Category | Examples |
|---|---|
| Prior expectations and learning | Previous negative experiences with treatments, learned associations |
| Socio‑demographic characteristics | Female gender, younger age, lower education |
| Personality and individual differences | Anxiety, pessimism, high trait suggestibility, neuroticism |
| Neurodegenerative conditions | Parkinson’s disease, Alzheimer’s disease |
| Inflammatory conditions | Chronic pain, irritable bowel syndrome |
| Communication and patient‑physician relationship | Negative framing of side effects, poor rapport |
| Drug characteristics | Route of administration, perceived invasiveness |
| Setting | Hospital environment, white‑coat effect |
| Self‑awareness | Heightened bodily awareness (interoception) |
The review also highlighted that a trusting relationship with the treating physician and clear, tailored treatment instructions can act as protective factors against nocebo effects.
5. Magnitude of the Nocebo Effect: What the Numbers Tell Us
The nocebo effect is not a marginal phenomenon. In clinical trials, adverse events in placebo arms are often of similar quality and quantity to those in the active drug arm, meaning that a substantial proportion of side effects are attributable to non‑drug‑related factors. For example, a systematic review and meta‑analysis of large phase‑III COVID‑19 vaccine trials found that 76% of reported systemic side effects were unrelated to the actual receipt of the vaccine.
In the context of lipid‑lowering therapy, particularly statins, up to 50% of adverse effects reported by patients may be attributable to the nocebo effect. These perceived side effects frequently lead to therapy discontinuation, increased cardiovascular risk, and misreported adverse drug reactions (ADRs).
5.1 Social Learning: A Potent Inducer
The 2024 meta‑analysis by Saunders et al. quantified the effect of social learning on nocebo outcomes:
- Effect size vs. no treatment: Hedges’ g = 0.74 (medium‑large)
- Effect size vs. neutral modelling: g = 0.42 (small‑medium)
- Effect size vs. explicit instruction: g = 0.46 (small‑medium)
These results demonstrate that watching another person experience side effects is a powerful and robust way to induce nocebo responses.
5.2 Vaccine‑Specific Findings
A 2025 case‑control study of allergy‑like nocebo events after COVID‑19 vaccination found that, among 1,038 participants, nocebo responses were positively associated with a history of allergy (OR 1.78) and receipt of the COMIRNATY vaccine (OR 1.60), and negatively correlated with perceived vaccine effectiveness (OR 0.93).
In contrast, a 2025 randomised clinical trial (the “LIFe Study”) comparing in‑depth versus concise medical briefings for COVID‑19 vaccination found no significant difference in adverse event scores between the two groups, suggesting that the nocebo effect was not a significant factor in that specific vaccination context. The study did, however, find that higher adverse event scores were associated with younger age and female gender.
6. Implications for Pharmacovigilance
The nocebo effect poses a two‑part problem for pharmacovigilance. As Austin Due articulated in a 2025 analysis, nocebo effects are (i) genuine patient harms that must be addressed ethically, and (ii) “noise” in pharmacovigilance databases that can distort safety signal detection.
6.1 Distortion of Adverse Event Data
When patients report symptoms that arise from negative expectations rather than drug pharmacology, those reports are entered into spontaneous reporting systems (e.g., FAERS, EudraVigilance, VigiBase) as suspected ADRs. This “noise” can:
- Inflate the apparent incidence of certain adverse events for a given drug.
- Mask true safety signals by obscuring the relationship between drug and event.
- Trigger unnecessary regulatory actions based on artefactual signals.
- Divert pharmacovigilance resources away from genuine safety concerns.
A 2025 commentary from DrugCard emphasised that nocebo effects can mimic real ADRs, complicating safety signal detection, and that misattributed symptoms may contribute to inflated adverse reaction data.
6.2 Impact on Clinical Decision‑Making and Adherence
Perhaps even more critically, nocebo effects can lead patients to discontinue essential medications. The statin example is instructive: up to half of reported statin side effects may be nocebo‑driven, yet discontinuation of statins increases cardiovascular risk. A 2025 case report described a 65‑year‑old woman who experienced dose‑dependent left‑sided abdominal pain with multiple different statins. When subjected to a six‑week, single‑blinded, placebo‑controlled crossover test using neutral containers, she reported comparable intermittent pain during both the placebo and atorvastatin phases. Once informed of the result, she resumed atorvastatin and her symptoms gradually subsided.
This case illustrates a key pharmacovigilance lesson: not every reported ADR is truly drug‑induced. Blinded provocation testing can help separate mind‑driven responses from genuine drug effects, though it is rarely performed outside research settings.
6.3 Ethical Considerations
The nocebo effect also raises ethical questions about informed consent and risk communication. Clinicians and regulators face a dilemma: how to fulfil the ethical obligation to disclose potential side effects without inadvertently inducing those very symptoms. Withholding information violates patient autonomy, while full disclosure can create nocebo harms. Striking the right balance requires evidence‑based communication strategies—a topic addressed in the next section.
7. Mitigating the Nocebo Effect: Evidence‑Based Strategies
7.1 Optimising Risk Communication
The way side effect risks are communicated has a profound impact on whether they are experienced. A 2025 study protocol for a randomised controlled trial of lumbar puncture (LP) noted that “merely disclosing the risk of side effects and complications of treatments or medical procedures increases reports of adverse events” and that headache after LP is “caused to a large degree by the information delivered by the physician during the informed consent procedure”. The trial is testing whether optimised communication strategies—designed to minimise nocebo effects during informed consent—can reduce headache‑related impairment after LP.
Practical communication strategies include:
- Positive framing: Instead of saying “this drug may cause nausea in 10% of patients,” say “90% of patients do not experience nausea.”
- Attribute normalisation: Explain that some symptoms (e.g., mild fatigue) are common in everyday life and may not be drug‑related.
- Individualised risk assessment: Tailor the disclosure of side effects to the patient’s specific risk profile, rather than providing a generic list.
- Use of neutral language: Avoid alarmist terms such as “dangerous,” “severe,” or “toxic” unless truly warranted.
7.2 Positive Social Modelling
If observing others experiencing side effects can induce nocebo effects, then observing others not experiencing side effects may be protective. A 2025 study investigated whether a positive social modelling intervention could reduce nocebo side effects. Participants were told they were testing a new cognitive enhancer (actually placebo) and received side effect warnings. Those who viewed a video of a peer reporting a positive experience with no side effects had significantly lower symptom severity than those who did not. The authors concluded that positive social modelling reduces nocebo side effects and may be an effective method to mitigate their burden in clinical settings.
7.3 Enhancing the Patient‑Provider Relationship
A main pathway of placebo and nocebo effects occurs via communication in patient‑provider interactions. A trusting relationship with the physician, clear explanations, and empathetic communication act as protective factors against nocebo effects. Training clinicians in communication techniques that avoid nocebo language and enhance placebo potential is a promising avenue for intervention. Advances such as virtual reality (VR) training and e‑learning modules are being developed to help clinicians practice these skills.
7.4 Educational Interventions for Patients
Simply educating patients about the nocebo effect can reduce both the incidence and severity of adverse effects. A 2025 study found that informing participants about the nocebo effect reduced nocebo side effects, and 86% of participants evaluated the nocebo information as helpful in general.
8. Latest Research Highlights (2024–2026)
8.1 Neuroimaging and Persistence of Nocebo Effects
The 2024 Scientific Reports study discussed earlier is notable for demonstrating that nocebo effects are remarkably stable over time and that their persistence is predicted by distinct neural activity (thalamus). This has implications for clinical practice: once a nocebo expectation is formed, it may not simply fade away without active intervention.
8.2 COVID‑19 Vaccination and the Nocebo Effect
The COVID‑19 pandemic provided a natural laboratory for studying nocebo effects at scale. The Mayo Clinic Press has noted that a substantial proportion of vaccine side effects are not actually caused by the vaccine ingredients but are the result of negative expectations. The 2025 LIFe Study, however, found that the nocebo effect was not a significant factor in COVID‑19 vaccinations when comparing different styles of medical briefing. This suggests that context matters: in high‑stakes, widely publicised vaccination campaigns, other factors (e.g., media coverage, social norms) may dominate.
8.3 The High‑Risk Model of Threat Perception
A 2025 review by Ian Wickramasekera introduced the High‑Risk Model of Threat Perception (HRMTP) to explain how threat activation in the brain‑body is associated with learned nocebo and placebo effects induced by fake invasive medical‑surgical procedures. The model postulates 10 psychosocial risk factors that modulate threat perception, driving nocebo somatic effects and functional somatic disorders. Importantly, the model predicts that threat activation by unconscious neutral stimuli can amplify nocebo effects and negate placebo effects in clinician‑patient relationships.
8.4 Generative AI and Nocebo Effects
A 2025 viewpoint article in JMIR Mental Health explored the implications of generative AI (e.g., chatbots) for clinical communication. The authors noted that as AI‑mediated interactions become more common, there is a risk that poorly designed AI could inadvertently strengthen nocebo effects. Conversely, if designed with psychosocial awareness, AI could be used to reinforce positive expectations.
9. Future Directions
Despite significant advances, several key questions remain unanswered:
- How can nocebo effects be reliably distinguished from true ADRs in routine clinical practice? Blinded provocation testing is currently the gold standard but is resource‑intensive.
- What is the optimal framing of side effect information for informed consent? A balance must be struck between transparency and the prevention of iatrogenic harm.
- Can nocebo effects be “inoculated” against through pre‑exposure to positive information? The positive social modelling studies suggest this may be feasible.
- How can pharmacovigilance databases be adjusted to account for nocebo “noise”? Statistical methods that incorporate expectation‑related factors are an area of active research.
- What role will AI play in shaping patient expectations? As AI becomes more integrated into healthcare, its potential to induce or mitigate nocebo effects must be carefully managed.
10. Conclusions
The nocebo effect is a genuine, neurobiologically mediated phenomenon that can cause real harm to patients and distort drug safety data. It arises from negative expectations, classical conditioning, and observational learning, and is influenced by a range of risk factors including prior negative experiences, personality traits, and the quality of patient‑provider communication.
For pharmacovigilance, the nocebo effect presents a two‑part challenge: it generates genuine patient harms that require ethical attention, and it introduces “noise” into ADR databases that can confound signal detection and lead to unnecessary treatment discontinuation.
Fortunately, evidence‑based strategies exist to mitigate nocebo effects. These include optimising risk communication (using positive framing and normalisation), employing positive social modelling, enhancing the patient‑provider relationship, and directly educating patients about the nocebo phenomenon itself.
As the latest research from 2024–2026 demonstrates—from neuroimaging studies to large vaccination trials—the nocebo effect is not an obscure curiosity but a central factor in medication safety and clinical outcomes. Understanding it is not merely an academic exercise; it is essential for every healthcare professional who prescribes medications, obtains informed consent, or monitors drug safety.
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